We plan to explore the interactions between coagulation factors and platelets. We will isolate the vitamin K dependent factors from frozen human plasma, iodinate them with 125I and activate them. We will investigate the mechanism of platelet surface prothrombin activation, the nature of the Factor Xa receptor site and the role of platelet coagulation Factor V. We will also determine whether Factor X is activated on the platelet surface and what reactions take place between Factors VII, IX, X. Using our recently isolated human Factor V we will prepare 125I-Va and measure its binding to the platelet surface prothrombin activation sites, comparing binding to that for Factor Xa, using our monoclonal antibody to the glycoprotein that is absent in thrombasthenia. We will isolate and characterize this glycoprotein and attempt to determine its role in platelet aggregation and clot retraction.